Many areas of the world do not have adequate fresh water supplies but they are near seawater. Seawater can be desalinated using reverse osmosis (RO). During RO, the feed water must be pressurized above the osmotic pressure of the feed water. The feed water becomes concentrated during this process and its osmotic pressure increases. Feed water pressures for seawater reverse osmosis (SWRO) are typically in a range of 50-70 bar (approximately 725 psi to 1015 psi).
Pressurizing the seawater in an RO system consumes energy. One approach to reduce energy consumption is to recover energy from the residual pressure of the brine after it leaves an RO module. An energy recovery pumping system is described by Childs et al. in U.S. Pat. No. 6,017,200 entitled “Integrated Pumping and/or Energy Recovery System.” This approach uses multiple water cylinders moving in a phased relationship to provide pressurized feed water to a RO membrane unit. One side of a piston in the water cylinder drives the feed water to the RO membrane unit while the other side of the piston receives brine from the RO membrane unit. The pressure of the brine reduces the power required to move the piston. Each water cylinder is connected to a separate hydraulic pump and hydraulic cylinder combination to move the piston in the water cylinder according to a desired velocity profile and to provide the additional energy required to pressurize the feed water.
U.S. patent application Ser. No. 13/250,463, entitled “Energy Recovery Desalination”, by D'Artenay et al. describes an energy recovery pumping system that makes various improvements to the Childs et al. system. For example, each of the hydraulic pumps has an adjustable swash plate to change the rate and direction of hydraulic fluid flow to its associated hydraulic cylinder. Inner and outer control loops are used to modify the position of the swash plate so that the water cylinder connected to the hydraulic cylinder follows an intended velocity profile more closely.